The goal of this project is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. Toxicity testing is an important aspect of public health research in that it serves to identify chemicals that are hazardous to human health. Proper conduct of toxicology studies is required to ensure that the resulting data are reliable. This project involves the conduct of method development work to support the design and conduct of toxicity and carcinogenicity studies of multiple, diverse mineral fibers in rats, following exposure by nose-only inhalation. There are concerns over sources of these mineral fibers across the USA, and there are questions surrounding similarities among relative potencies. The data from this project will be used in the development of sound, scientific conclusions about the potential toxicity and carcinogenicity of naturally occurring asbestos and related mineral fibers in rats and ultimately supports the risk assessment efforts of National Toxicology Program and other federal agencies. In addition, the public can be adequately informed about risk factors arising from exposure to naturally occurring asbestos and related mineral fibers. Keywords: toxicity, carcinogenicity, asbestos, inhalation, fiber, lung